Formation of chromium-containing coatings on steel strip



April 4, 1967 E. H. MAYER ETAL 3,312,546

FORMATION OF GHROMIUM-CONTAINING COATINGS ON STEEL STRIP Filed Oct. 20, 1965 3 Sheets-Sheet l INVENTORS Edward H. Mayer Richard M. Wi/l/son April 4, 1967 5 MAYER ET AL 3,312,545

FORMATION OF CHROMIUM-GONTAINING COATINGS ON STEEL STRIP Filed Oct. 20, 1965 3 Sheets-Sheet 2 Coating Interface Surface D/lsfance from surface in inches INVENTORS Edward H. Mayer Richard M. Wi/lison April 4, 1967 E. H. MAYER ETAL 3,312,546

FORMATION OF CHROMIUM-CONTAINING COATINGS 0N STEEL STRIP Filed Oct. 20, 1965 3 Sheets-Sheet 3 INVENTORS Edward H. Mayer Richard M. Wi/I/son United States Patent 3,312,546 FORMATION OF CHROMIUM-CONTAINING COATINGS 0N STEEL STRIP Edward H. Mayer and Richard M. Willison, both of Bethlehem, Pa., assignors to Bethlehem Steel Corporation, a corporation of Delaware Filed Oct. 20, 1965, Ser. No. 505,587 23 Claims. (Cl. 75-208) This invention relates to the formation of a chromiumcontaining coating on steel strip, and more particularly to the formation of an iron-chromium alloy coating.

This application is a continuation-in-part of application Serial No. 370,288, filed May 26, 1964 and now abandoned, which in turn is a continuation-in-part of application Ser. No. 297,461 filed July 24, 1963 and now abandoned.

The principal object of this invention is to producean adherent, protective iron-chromium alloy layer on the surface of steel articles such as strips, sheets, plates, rods, bars and wire.

It is extremely difiicult to produce a uniform ironchromium alloy layer on thin articles in the form of elongated strip. Ordinary chromizing processes, e.g. those in which chromium is used in conjunction with a halide, and usually packed around the article to be coated, are not satisfactory for treating strip, for in order to coat both sides of the strip, or even one side, the strip, if in the form of a coil, must be unwound and the coating material applied and the alloy formed during movement of the strip, or, alternatively, if the strip is coated while stationary, the coating must be performed in such great lengths as to require a heating unit of impractical size, or, if the coating is formed on the strip in small increments, the time consumed would render the process impractical.

We have found that by applying a uniform layer of chromium-containing metal alloying material, in the form of a relatively fine powder, to the strip, and following this with certain controlled treatment steps, a thin, adherent, ductile and corrosion resistant coating is produced on the surface of the strip.

In accordance with this invention, a steel sheet or strip is preferably coated with a thin film of liquid. The liquid should have such viscosity, volatility and tackiness characteristics as to render it suitable as a temporary bonding agent for subsequently applied metal powder. A chromium-containing metal powder is next applied uniformly over the surface of the strip, the powder being held in place by the previously applied liquid film. The powder covered strip is then subjected to a rolling operation, or equivalent pressure application, to compact the powder. In this step, the powder is rolled into a fiat, compacted metallic layer in which adjacent grains of powder are bonded together. This metallic film is in a semi-adherent condition in relation to the strip, the underside of the metallic film having been mechanically bonded to the strip surface. The composite article of strip and compacted metal powder is then sintered in a heat treating furnace at a controlled temperature, and for a time period sufficient to provide formation of an adherent ironchromium alloy on the surface of the strip. The sintering step is performed in a protective environment. Unless the strip and applied powder have a carbon content below a certain predetermined maximum, the composite article is decarburized before the sintering operation. The strip itself should have not more than 0.01% effective carbon by weight at the time of sintering. Effective carbon is hereinafter defined. The carbon content of the applied powder should be not more 0.25% by weight in order to produce a desired coating. However, in general, the lower the carbon content, the greater will be the thickness, ductility and corrosion resistance of the 3,312,546 Patented Apr. 4, 1967 coating. The decarburizing step can be avoided by selecting a base steel having a carbon content which meets the above-listed maximum requirement.

Where our method is applied to fiat rolled stock, such as sheet, strip or plate, the coating may be applied to one side or to both sides of the strip, as desired.

In the drawings:

FIG. '1 is a reproduction of a photomicrograph of a transverse section of a steel strip coated with an ironchromium alloy.

FIG. 2 is a reproduction of the line scan of X-ray intensity for chromium in a rolled powder coating after sintering, as determined by electron microprobe.

FIG. 3 is a reproduction of a macro photograph of a transverse section of a coated strip in which one end of the section has been acid etched to expose the coating.

In one specific embodiment of the invention, a 20 gage strip of rimmed steel which has been decarburized to a carbon content of 0.003% is unwound from a coil, and a thin film of tridecyl alcohol is applied to both sides of the strip by means of rubber rolls. The strip then passes under a vibrating dispenser from which finely divided ferrolls, where the powder is compacted with the base metal strip at a pressure sufficient to produce a strip elongation of 10%. After leaving the rolls, the strip is wound on a take-off reel. Before heat-treating, the compacted, coiled strip is unwound and recoiled. During recoiling, spacers are inserted to produce an open-coil effect. It is preferable to space the laps of the coil to permit the sintering atmosphere to circulate freely, and to prevent possible welding of adjacent laps. The spaced, or open, coil is placed in an annealing furnace, where" it is heat treated at 1650 F. for 48 hours in a dry atmosphere of hydrogen. After sintering, the strip is brushed with brightening brushes to remove any unsintered powder from the surface of the strip. The brushed product has a bright metallic lustre.

As a modification of the previously described process, a steel strip having the same characteristics as the strip of the previous example is filmed with tridecyl alcohol as before. The powder used is a mixture of iron and chromium powder having a fineness of mesh. In this example, the strip is passed vertically in an upward direction through a chamber containing a fluidized bed of the powder. The fluidized bed is maintained by forcing air through a porous diaphragm at the bottom of the chamber, the forced draft keeping the powder in a state of suspension. The strip passes through a narrow opening in the diaphragm and then through the fluid bed. By the time it leaves the top of the closed chamber, the strip is uniformly coated with the metal powder. The coated strip is rolled to compact the powder on the strip, coiled under minimum tension to prevent welding during sintering, and then sintered at 1650 F. for 48 hours. After brushing, the strip presents the same metallic lustre as does the finished strip of the first example.

In a further modification, a strip of cold rolled rimmed steel is used as the base metal or backing layer. After filming the strip with tridecyl alcohol, a metal powder mixture of a fineness of l50 mesh is distributed uniformly on one side of the strip surface by means of a vibrator dispenser. The powder in this instance is a mixture of ferrochrome alloy (70.0% Cr) powder mixed a) with about of a commercial nickel powder. The powder is compacted on the strip by rolling through 4 in. diameter steel rolls. The strip, wound after rolling, is unwound and recoiled with spacers applied, preparatory to sintering. The coiled strip, bearing spacers, is placed in an annealing furnace, and, because of the carbon content of the strip (0.06% C) it is given a decarburizing treatment in a moist hydrogen atmosphere (dew point, +95 F., or 5.5% by volume). The strip is decarburized at 1250 F. for about five hours to bring the carbon content of the steel base to 0.01%, or below. After decarburizing, the moist hydrogen atmosphere is removed and replaced with a substantially pure, dry hydrogen gas, thus ensuring against oxidation. The compacted, decarburized strip is then annealed (sintered) at 1650 F. for 24 hours.

While in the foregoing examples, our invention has been described as a method for the formation of an iron-chromium alloy coating on a metal article in the form of a steel strip, it will be readily recognized that the invention has equal adaptability to the formation of such coating on steel sheet or plate.

When the invention is applied to sheet or plate, the same major processing steps, i.e. applying the powder, compacting and sintering, are used, the only difference being in the manner of physically handling the article. In the case of sheets for example, after they have been rolled to compact the powder, they should be placed in the annealing furnace in such a manner as to prevent welding during sintering.

In the appended claims, the designation steel strip is meant to include steel sheet or plate as well.

It should be noted that alternative methods, other than the ones described in the examples, may be used in the application of the powder to the strip. Such methods may include electrophoretic deposition and electrostatic spray technique, or any other method by which the powder can be effectively applied to the strip.

While it may be preferable to perform our process by starting with a steel which has been decarburized before any of the processing steps of the invention are applied, this becomes merely a matter of choice, as illustrated by the example in which a rimmed steel was decarburized after compacting and just prior to sintering. There is no limitation on the type of steel which may be used as base material in our invention, as long as the effective carbon content of the base metal is maintained at a figure no greater than 0.01% during sintering.

Most metal powder will contain some carbon, and this carbon must be held to a value which will not produce deleterious chromium carbides in the ultimate alloy coating. As previously stated, the permissible amount of carbon which may be introduced into the compacted article by the powder should be not more than 0.25% by weight of the powder used. In any event, the total effective carbon in both metal strip and powder should be sufiiciently low, so that the resultant alloy coating after sintering will be continuous, ductile and corrosion resistant.

The term effective carbon as used here refers to that carbon in either the base metal or the coating, which is available to produce chromium carbides. These carbides, if present in sufficient amount, embrittle the coating and thus limit the formability of the coated product. Furthermore, a coating containing chromium carbide has lower corrosion resistance than an alloy coating free of carbide.

If it is desired to incorporate the strength characteristics of a carbon steel in the coated strip, it is possible to use a strip analyzing more than 0.01% carbon, if a sequestering agent such as titanium is present in the strip. If the titanium is present in sufficient quantity to combine with substantially all of the carbon in the strip, it will prevent the carbon in the strip from diffusing to the coating during sintering and forming undesirable chromium carbide in the coating. Other carbide formers which may be used to tie up the carbon are chromium and columbium. When carbide formers are used in the strip to tie up the carbon in this manner, it is still essential that the unbound, effective carbon in solution in the strip, that which is free to react with the chromium in the compacted powder, be held to a quantity not in excess of 0.01%.

When the carbon in the base steel is higher than 0.01%, and titanum is used to combine with the excess carbon, the amount of titanium necessary will of course depend on the amount of carbon to be sequestered. As a practical matter, when using a titanium stabilized steel, it is preferred to maintain the carbon in an amount ranging from 0.03% to 0.06%. Titanium may be present in an amount ranging from 0.2% to 0.5%, preferably in the range of from 0.25% to 0.35%, and always in an amount at least four times the amount of carbon it is necessary to sequester.

As an example of the freedom of selection of the type of steel which may be used as base material, a cold rolled 20 gage strip of titanium stabilized steel has been employed as basestock. The strip had an analysis of 0.30% titanium, and 0.06% total carbon. This amount of titanium combines with substantially all of the carbon to form a stable titanium carbide.

In this example the strip is filmed with alcohol as in the previous examples, and a metal powder distributed uniformly on both sides of the strip. Ferrochrome alloy powder, analyzing 71.0% chromium, 27.5% iron and 0.08% carbon, is the powder applied. All of the powder passes a mesh screen, while approximately half of it passes a 200 mesh screen. The powder-bearing strip is drawn through the compacting mill, and the powder compacted with the base metal. The product from the compacting mill is then sintered at 1700 F. for 24 hours.

As most of the carbon in the strip has been sequestered by the titanium, with less than 0.01% of the carbon being free to diffuse into the coating, the resultant coating is free of the deleterious chromium carbides which develop pores, brittleness and inferior corrosion resistance in an alloy coating of this nature. The coated product, after being bent through degrees, will withstand immersion in boiling 20 volume percent nitric acid.

Generally, if the steel strip stock is soiled, it should be cleaned with a cleaning medium such as a hydrocarbon solvent or an alkali cleaner.

The powder retaining material, which is applied to the strip in the form of a thin film, and which acts temporarily as a powder-retaining medium, may be any liquid substance having the proper viscosity, volatility and tackiness characteristics previously referred to, and which, in addition, leaves no carbon deposit on the steel surface, and meets industrial safety requirements. The metal powder can be applied to the steel backing member without a liquid substance having the above-stated characteristics first being applied, but the use of such film is preferred, for the film lends mechanical efficiency to the powder applying and compacting steps.

While not critical, it is desirable to control both the amount of liquid applied and the grain size of the metal powder. The alcohol, or other substance, used for retaining the metal powder, should be applied in a rather thin film of a thickness just sufficient to cause adequate adherence of the powder particles. An excess of the liquid may cause problems of slippage and inefficient compacting during the rolling operation.

In selecting a particular liquid as the powder-retaining medium, care should be taken to select one which will not leave any substantial carbon deposit in the compacted metal, which in turn could produce brittleness. Kerosene is an alternative liquid which has been used successfully as the powder-retaining agent in the production of our alloy coatings. Transformer oil and straw oil have also been used, although with less efficiency than either tridecyl alcohol or kerosene. Other oils which may be used are naphtherie base oils having a Saybolt Universal viscosity of from 90 to 110 seconds at 100 F.

Metal powder which passes a 150 mesh screen has been found to be quite satisfactory, although larger particles may be used. The size of powder particles desired will depend somewhat on the type of liquid film used, and on the physical manner in which the powder is applied.

If the powder is applied to the strip in an amount of from 8 to 10 grams per square foot of backing strip surface, quite satisfactory results are obtained, and this amount of powder is held on the strip readily by a very light film of tridecyl alcohol. Heavier or lighter applications of powder may be used, depending to some extent on the desired distribution of chromium and the thickness of the sintered coating. By using a heavier alcohol film, the amount of powder applied can be increased by approximately a factor of two, over the 10 grams per sq. ft. figure given above.

The following table lists data drawn from a number of examples to show the quantities of alcohol and of powder, and their relationship to each other, from which, in certain examples, satisfactory coatings have been made, and where, in other examples, either the quantity of alcohol, or of powder, has been too heavy to form a uniform coating.

In compacting the powder on the strip, the desideraturn is to provide a uniform metal shell on the base metal where individual powder particles have been welded together, and particles immediately adjacent the base metal have been forced mechanically into the base metal surface. Excellent compacting has been obtained with chromium-bearing metal powders by applying a roll pressure which elongates the base metal by from 1% to 10%. These figures are not to be construed as limiting, but merely as a guide, for individual operators will vary the pressure to suit differences in operating conditions, such as hardness of base metal, size of rolls, grain size of powder, etc.

The compacted shell of powdered metal is porous in nature, and this is shown, advantageously, in the subsequent sintering step. During the heating-up period prior to sintering, the volatile oily material, originally applied to hold the powder to the strip, is vaporized and escapes through the pores of the compacted layer. If the compacted layer were not porous, the layer would be lifted off by the vapors during sintering.

The temperature during sintering should preferably range between approximately 1550 F. and 1900 F. for not less than 12 hours, although considerably longer times may be desirable, depending on the amount of alloying required. Actually, there is no upper limit for sintering temperature other than that which may be dictated by practical considerations. At temperatures above 1550 F., the minimum time required will be lowered in an inverse manner.

When decarburizing is performed in the annealing vessel, just prior to sintering, the holding temperature may range from 1150 F. to 1450 F. For example, a dccarburizing time of about twelve hours at the minimum temperature of 1150 F. will ordinarily be satisfactory with strip containing approximately 0.08% carbon.

Illustrative of the time-temperature cycle permissible is one example wherein a 20 gage strip of rimmed steel, having a carbon content of 0.002%, is filmed 'with alcohol, and coated with a powder comprising 50% H- iron (99.2% Fe, hydrogen reduced) and 50% electrolytic chromium. After compacting the powder on the strip, the compacted product is sintered at approximately 1850 F. in a pure hydrogen atmosphere for a period of 12 hours. After cooling, the alloy coating, produced during this shortened sintering period, is subjected to analysis by electron microprobe. Results of such analysis indicate approximately 12% chromium at the area immediately adjacent the interface between the coating and the steel base, with an increasing chromium gradient in the direction of the coating surface, the chromium content being 24.0% at the surface.

The different types of powders contemplated for use in this invention are those containing chromium, iron and chromium, or iron and chromium and nickel, where chromium is present in suflicient quantity to produce an ironchromium alloy in the coating of the finished product. Metal powders answering this description are iron-chromium alloy, a mixture of iron and chromium, commercial grade chromium powder, iron-chromium alloy plus nickel, a mixture of chromium and nickel powders, -ironchromi um-nickel alloy and a mixture of iron, chromium and nickel powders. In the case of the alloy powders, it will be apparent that iron, chromium or nickel powder may be added if desired.

The expression a chromium-containing powder, as used in the ensuing claims, means a powder containing chromium in sufficient quantity to produce an iron-chromium alloy in the coating of the finished product, in which powder there may also be presentiron or nickel, or inert material or other substance not adversely affecting the process.

Utilization of a powder composition comprising 20% Cr and Fe will develop a stainless steel type coating which will resist a boiling 20 volume percent aqueous solution of nitric acid (based on HNO Use of powders containing any increasing amount of chromium, over the 20% example just given, and up to 100% chromium, will likewise produce a coating resistant to 20% nitric acid. Other mixes of powders which can be used to form coatings resistant in the nitric acid test, wherein the chromium is present in an amount not less than 20 weight percent, are:

Chromium-I-nickel Ferrochrome alloy-l-nickel Chromium-l-nickcl-l-iron The coating which results from our invention will generally have a thickness of from about 0.001 inch to 0.003 inch, and it is a true alloy of iron and chromium, or of iron, chromium and nickel. The iron-chromium content, or the iron-chromium-nickel content, of the alloy will depend on the amount and analysis of the powder used, as well as on the sintering time and temperature.

From the point of view of carrying out the steps of the process, the chromium content of the applied coating i immaterial; i.e., the process itself can be effected with powders of low chromium content or with powders of high chromium content. However, the chromium content of the powder has a marked effect on the nature of the coating produced by the process.

If the chromium content of the powder is less than about 20%, say about 15%, sintering will produce an iron-chromium alloy coating in which the chromium content will range from say 10% at the outer surface of the coated sheet to zero percent towards the interior. The chromium content at the outer surface, and the depth of the chromium-containing alloy layer will depend on sintering time and temperature.

If, however, the chromium content of the applied powder is equal to or greater than about 20%, a coating can be produced on the steel which is a stainless alloy of chromium and iron containing not less than about 12% .Cr throughout and characterized by a sharp interface between the alloy coating and the metal therebelow. Beneath the interface, the chromium content of the steel drops rap-idly to zero.

Preferably, our coating has an average chromium content ranging from about 15% to about 25%. Higher chromium contents may be used, but there would probably be little or no added benefit from the standpoint of corrosion resistance, and the added cost would not be warranted.

The mechanism by which this stainless alloy layer is formed is believed to be as follows:

Sintering of the compacted metal powder coating occurs by diffusion of iron and chromium atoms. This diffusion first causes bonds or bridges of continuous metal between individual powder particles and also between powder particles and the base steel. This action produces voids between particles. As diffusion progresses it reduces the surface area of the voids to a point where the voids are very small, or do not exist. If the chromium content of the coating is greater than about 12%, the structure of the sintered compact is body-centered cubic, or ferritic, at all temperatures between room temperature and the diffusion temperature. Below about 12% chromium the structure is face-centered cubic, or austenitic, at the diffusion temperature of 1600 F., but is bodycentered cubic at room temperature. Diffusion at any given temperature is faster in ferrite than it is in austenite.

The gradient in chromium content of the alloy layer and the thickness of the alloy layer can be varied as desired.

With a short sintering time, e.g. hours at 1750 F. the chromium content of the alloy layer will range from about 12% at the interface to a quantity approximately that of the applied powder at the outer surface, and the interface will be only slightly below the original surface of the steel base.

With longer sintering times, more iron can migrate to the outer surface of the article, the chromium content of the alloy layer will tend to become more uniform throughout, and the interface will be somewhat lowered, i.e., the thickness of the alloy layer is somewhat greater. If sintering times are prolonged unduly the stainless alloy layer will disappear, i.e. diffusion of chromium and iron will continue to take place until the chromium in the outer coating is less than 12%. This, however, would require sintering times of such duration that they would be far beyond any time period consistent with practical operation.

Illustrative of the type of stainless coating that can be produced by this process is the following example.

A steel strip with a carbon content of 0.003% was wet with alcohol and there was applied to the strip a coating of iron-chrominum alloy powder analyzing 70% Cr of 0.001" thickness. The coated strip was put through rolls and elongated to compact the coating. The coated strip was sintered at 1750 F. for 48 hours. Microprobe examination of the strip showed that the strip was coated with a stainless iron-chromium alloy, the chromium analysis being illustrated oy the line scan shown in FIG. 2.

As a further example, a strip of steel of 0.003% carbon content was coated in the same manner with a powder analyzing 75% Cr and 25% Ni. The strip was rolled, etc. and sintered at 1750 F. for 48 hours. A coating was produced which consisted of an iron-chromiumnickel alloy having a carbon content of 0.05%. The microprobe analysis for chromium was substantially the same as that shown in FIG. 2.

It will be seen from these examples that by this process it is possible to produce on steel strip a coating of ironchromium alloy characterized by a chromium content of about 12% or greater and a carbon content not in excess of 0.10%, which coating is stainless and ductile and 8 metallurgically bonded to the base. of this type will resist a 20% boiling nitric acid solution. This is a standard test for estimating corrosion resistance of an iron-chromium alloy of the stainless type.

The sintered coating has a dull grey appearing surface lacking in metallic lustre. The coated strip can be used in this condition, or it can be brushed or buffed to improve the surface appearance. However, the strip will usually be given a skin rolling of from 1% to 2% elongation to improve the mechanical properties. If the coated stri is to be used for decorative purposes, a combination of skin rolling with polished rolls and brushing, bufiing or other means of brightening the surface may be used to produce a surface with high lustre.

Sintering must be performed in a protective atmosphere or environment, substantially free of carbon, oxygen or nitrogen. To this end, any one of the noble gases may be used as a surrounding atmosphere, although a more practical atmosphere is one composed of substantially pure hydrogen. Hydrogen has the added advantage of being able to remove oxygen from oxides which may have formed during processing. As an alternative, sintering may be performed in a vacuum, either complete or partial, using a hydrogen or noble gas atmosphere.

The hydrogen gas used to promote a reducing atmosphere in the sintering furnace should preferably be substantially 100% pure hydrogen. However, even when allegedly pure hydrogen is used, certain impurities may enter the furnace atmosphere, in large scale operations, through leaks in the system, from the furnace walls or other portions of the equipment or, possibly, in the hydrogen employed. Chromium has a strong affinity for carbon, nitrogen and oxygen, any one of which might find its way into the sintering furnace as impurity. The impurity most likely to be found in the sintering atmosphere, and the one which normally will be the chief source of trouble in a sintering operation, is oxygen.

At the sintering temperature, and even considerably below such temperature, the highly reactive chromium powder reacts with any small amount of oxygen present in the atmosphere, and the resultant chromium oxide may completely surround the exposed portions of the powder particles. The formation of the oxide shell on the particles hinders the normal diffusion of the chromium particles into the steel base. For this reason, When oxygen is present as impurity in the sintering atmosphere, it may be necessary to provide a means whereby the chromium powder is freed of its oxide, and can then diffuse with the iron, both in the steel strip base, and in the powder itself in the case where there is iron in the powder.

Inclusion of a halogen-containing material in the furnace atmosphere, has been found to promote the rapid diffusion of chromium into the iron, and contrariwise, the iron into the chromium, when oxygen impurities are inadvertently introduced into the furnace. Halogens, or halogen compounds, act as scavengers, or energizers, in that they remove the oxide film from the chromium powder particles, ensuring metal to metal contact. The use of a halogen-containing material promotes the diffusion rate to such an extent that, it has been found, in some instances, a halogen or halide, as energizer, reduces the sintering time to as little as four hours. Furthermore, when a halogen or halide energizer is used in the furnace, a higher carbon content is permissible in the compacted powder. In addition, alloy coatings made by our process in a commercial annealing furnace, where an energizer was introduced into the sintering atmosphere, proved considerably more ductile than coatings made in the absence of energizers. For example, in one instance, where a halogen-containing energizer was used, the resultant product, of chromium alloy coating on a low carbon steel strip base, was rolled on a reducing mill to a total reduction of the cross-sectional area of approximately percent, or from a thickness of 0.057 inch to 0.0115 inch. Without the use of energizers, the product can be reduced not more than 50 percent.

A stainless coating After reduction of the thickness of the coated strip, in the range of from to 80 percent, the ductility will be reduced to the point where use of the product will be limited to those applications requiring a minimum of deformation. If additional formability is required for the intended application, the product can be annealed after cold rolling to restore its ductility.

While any halogen or halogen compound may be used as the energizer, which is volatile at the sintering temperature, or a few hundred degrees Fahrenheit below the sintering temperature, it is preferable to use as the energizer, one which can be introduced as a gas at a relatively low temperature. When the energizer is introduced in the gaseous form, the amount and rate of introduction can be closely controlled. Hydrogen chloride gas and hydrofluoric acid gas, among other halogencontaining vapors or gases, may be used for this purpose. Suitable halogen-containing materials, in the form of solid compounds which may be inserted into the furnace in solid form, and which volatilize at or near sintering temperatures, include ammonium chloride, chromic fluoride, and ammonium bifiuoride. Chlorine gas has been found to be especially advantageous when injected in gaseous form into the hydrogen atmosphere in the sintering furnace.

The manner in which a halogen-containing material can be utilized, in the preparation of a chromium alloy coating by our process, will be clearly shown in the examples which follow.

A 40 pound coil of 5 inch wide strip of decarburized rimmed steel was coated with a film of tridecyl alcohol in a roller coater. The filmed strip was then passed through a fluidized bed of low carbon ferrochrome powder (0.08% carbon, 70.7% chromium, 0.43% silicon and 0.50% manganese, balance iron) to form a uniform coating of the powder on the strip. The thus-coated strip was passed through a temper mill to compact the powder with the strip. Next, the strip was rewound with a spacer between laps, to produce an open-wound effect. The open-wound coil was placed in a retort along with a container bearing 50 grams of ammonium chloride powder. After insuring that the retort was tightly sealed, it was placed in a 16 inch diameter retort furnace, and the charge of strip and ammonium chloride heated to 1700 F., and maintained at this temperature for 24 hours. During the heating-up period and the 24 hour soak period, hydrogen gas was introduced into the system at the bottom of the retort at a rate of 15 cu. ft. per hour. At the end of the soak, or sintering, period, the charge was cooled to room temperature. There was a continuous flow of hydrogen through the retort during the cooling period.

In order to determine the efficiency of hydrogen chloride gas as'an energizer, a test was made in which 8 samples of 0.003% carbon sheet steel, each 4 inches by 10 inches and 0.057 inch thick, were filmed with tridecyl alcohol and coated with low carbon ferrochrome powder (70.5% Cr, 0.04% C and 0.45% Si). The powder was compacted on the strip by passing the sheets through a mill having 4 inch diameter rolls. Elongation after rolling was approximately 10%. The sheets were then placed in the retort of a retort furnace, and heated to 1700 F. in a stream of hydrogen. After the charge attained the 1700 F. temperature, it was maintained at this temperature for 24 hours, during'which period both hydrogen and hydrogen chloride were continuously passed through the retort. The hydrogen chloride was introduced at a rate representing 0.5% of the total atmosphere, while the hydrogen was introduced into the retort at a rate of cu. ft. per hour. At the end of the 24 hour period, the hydrogen chloride was turned OE and the charge cooled to room temperature. During cooling, the hydrogen flow through the retort was continued.

At the end of the run the specimens were found to have thick, continuous coatings, with an average thickness of 0.00115 inch. The coatings were uniform and extremely ducti e.

In another example, a 6000 pound coil of 36 inch wide strip of decarburized capped steel was coated with tridecyl alcohol by passing the strip through a roller coater. The strip was then coated uniformly with iron-chromium alloy powder by passing the strip through a fluidized bed of Simplex ferrochrome powder. This powder had an analysis of 71.3% chromium, 0.01% carbon and 1.46% silicon. The powder was compacted on the strip by rolling the powder-carrying strip to approximately 2% elongation on a temper mill having 28 inch diameter rolls. In preparation for the subsequent annealing step, the coil was open-wound by inserting spacers between the laps of the coil. The coil was then placed on a Lee-Wilson open coil annealing base, along with a container of pounds of pelletized chromium fluoride (CRF .3 H O). The pellets had a diameter ranging from to 1 inch. The charge, comprising the coil and container of pellets, was covered with an alloy steel inner cover. A Lee-Wilson gas-fired radiant tube annealing furnace was superimposed over the inner cover, and the charge was heated gradually to 1700 R, where it was held for 32 hours, after which the furnace was removed, and the charge and inner cover were allowed to cool. During the heating-up, sintering and cooling, hydrogen gas was introduced into the furnace in a continuous flow. During heating-up and cooling the rate of flow of hydrogen was 900 cu. ft. per hour, while during the sintering, or soaking period, the flow rate was reduced to 300 cu. ft. per hour.

By the foregoing method, coatings can be produced which are uniform and continuous with an average coating thickness of about 0.002 inch. Specimens of the coated strip resist boiling 20% nitric acid.

Because of the poisonous and corrosive nature of the halogens, proper precautions should be taken to prevent escape of these materials into the ambient atmosphere, by using approved dispensing equipment and by installing proper venting facilities on the sintering furnace.

Care should be exercised in the use of halogens to avoid formation of an explosive mixture with hydrogen. For example, in Bureau of Mines Bulletin No. 503, entitled, Limits of Flammability of Gases and Vapors, it is shown that chlorine and hydrogen are known to produce an explosive mixture when the chlorine content of the mixture is above 11.0%. However, there is never any need for using any halogen-containing material in such large quantity in our method. For practical operations, a halogen content between 0.10% and 1.0% has proved satisfactory. As indicated above, if the oxygen content of the furnace is extremely low, or non-existent, as may be, use of a halogen-containing material in the sintering atmosphere is not essential, but to insure formation of a commercial product having optimum properties of ductility, continuity and corrosion resistance, use of a halogen or halide gas in the sintering zone is very desirable.

While we do not wish to be bound by any theory upon which the success of our process may be based, it has been shown by analysis that the coating produced is made up entirely of iron-chromium alloy, or when nickel is used also as a starting material, of an iron-chromiumnickel alloy. This conclusion is reached through metallographic, X-ray diffraction and electron microprobe analysis of the base material and the coating of the sintering product. It is believed that the alloy is formed by a metal to metal diffusion, of iron and chromium, or of iron, chromium and nickel, so that at the completion of the sintering operation, a coating of an iron-chromium alloy, containing at least about 12% chromium, is formed. The metallographic structure of the sintered product is shown in the drawing in FIG. 1 where a represents the coating structure and b that of the base. The specimen shown in FIG. 1 was etched in Nital and reproduced at 250 magnifications.

That the coating is a discrete material is graphically illustrated in FIG. 3, which is a macro photograph taken l l. at magnifications. The base material has been partially etched away with HNO at one end of the specimen, leaving the coating untouched. From observation of FIG. 3, the thickness, ductility and corrosion resistance of the coating are readily apparent.

The term halogen-containing gas, as used herein, refers to a gas which comprises either a halide or a halogen.

All percentages given herein and in the appended claims, except those related to the volume of nitric acid and the volume of halogen-containing gas, refer to weight percent.

As has been fully described above, the stainless coating of this invention is produced through inter-diffusion of chromium into the steel base and of iron from the base into the compacted powder during sintering. In the appended claims, the stainless coating relates to a coating formed by such diffusion during sintering.

We claim:

1. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective environment for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

2. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective atmosphere for a time and at a temperature sufiicient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

3. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in an atmosphere of substantially pure hydrogen for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

4. The method of forming a coating on steel strip which comprises applying to thesurface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective atmosphere including a halogencontaining gas for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

5. The method of forming a coating on steel strip which comprises applying to the surface of a strip containing a carbide-forming metal in an amount sufficient to maintain the effective carbon in the strip at not over 0.01%, a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective environment for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating, and maintaining the effective car- 12 bon content of said strip at not more than 0.01% during said sintering.

6. The method of forming a coating on steel strip which comprises applying to the surface of a strip containing a carbide-forming metal in an amount sufficient to maintain the effective carbon in the strip at not over 0.01%, a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective atmosphere for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

7. The method of forming a coating on steel strip which comprises applying to the surface of a strip containing a carbide-forming metal in an amount sufficient to maintain the effective carbon in the strip at not over 0.01%, a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in an atmosphere of substantially pure hydrogen for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

8. The method of forming a coating on steel strip which comprises applying to the surface of a strip containing a carbide-forming metal in an amount sufficient to maintain the effective carbon in the strip at not over 0.01%, a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective atmosphere including a halogen-containing gas for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

9. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel, which powder contains not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective environment for a time and at a temperature suflicient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating, and main-taining the effective carbon content of said strip at not more than 0.01% during said sintering.

10. A method according to claim 2 in which the metal powder is a member of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel.

11. A method according to claim 3 in which the metal powder is a member of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel.

12. A meth-od according to claim 4 in which the metal powder is a member of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel.

13. A method according to claim 5 in which the metal powder is a member of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel.

14. A method according to claim 6 in which the metal powder is a member of the group consisting of chromium,

chromium-iron, chromium-nickel and chromium-ironnickel.

15. A method according to claim 7 in which the metal powder is a member of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel.

16. A method according to claim 8 in which the metal powder is a member of the group consisting of chromium, chromium-iron, chromium-nickel and chromium-ironnickel.

17. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a protective atmosphere including a halogencontaining gas in which the halogen represents not less than 0.10% by volume of said atmosphere for a time and at a temperature sufficient to cause diffusion between the strip and the powder and to thereby form an adherent stainless steel coating containing not more than 0.10% carbon, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

18. The method according to claim 8 in which the halogen of the halogen-containing gas represents not less than 0.10% by volume of the protective atmos here.

19. The method according to claim 17 in which the halogen-containing gas is hydrogen chloride.

20. The method according to claim 17 in which the halogen-containing gas is ammonium chloride.

21. The method according to claim 17 in which the halogen-containing gas is chromium chloride.

22. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, sintering the strip and compacted metal powder in a sintering zone in a hydrogen atmosphere, introducing into said atmosphere during the early stages of the sintering operation a halogen-containing gas in which the halogen represents not less than 0.10% by volume of the sintering atmosphere, removing said halogen-containing gas from said sintering zone before sintering is completed, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

23. The method of forming a coating on steel strip which comprises applying to the surface of a steel strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, heating the strip and compacted powder to a temperature not less than 1550 F. for not less than twelve hours in a continuously flowing hydrogen atmosphere, introducing into said atmosphere at halogen-containing gas in which the halogen represents between 0.10% and 10.0% by volume of said atmosphere during at least the early stages of heating, and cooling the heated strip in a continuously flowing hydrogen atmosphere free of halogen to a temperature at least at low as 500 F.

References Cited by the Examiner UNITED STATES PATENTS 1,365,499 1/1921 Kelley 29196.6 1,853,369 4/1932 Marshall 29l96.6 2,622,043 12/1952 Roush 1l7l07.2 2,791,517 5/1957 Becker et al. 117107.2 2,851,375 9/1958 Samuel 117-50 3,093,556 6/1963 Machu et al. 29-196.1 X 3,222,212 12/1965 Samuel et al. 117107.2

CARL D. QUARFO'RTH, Primary Examiner. BENJAMIN R. I ADGETT, Examiner.

M. I. SCOLNICK, Assistant Examiner. 

1. THE METHOD OF FORMING A COATING ON STEEL STRIP WHICH COMPRISES APPLYING TO THE SURFACE OF THE STRIP A METAL POWDER CONTAINING NOT LESS THAN 20% CHROMIUM AND NOT MORE THAN 0.25% CARBON, COMPACTING THE POWDER ON THE STRIP, SINTERING THE STRIP AND COMPACTED METAL POWDER IN A PROTECTIVE ENVIRONMENT FOR A TIME AND AT A TEMPERATURE SUFFICIENT TO CAUSE DIFFUSION BETWEEN THE STRIP AND THE POWDER AND TO THEREBY FORM AN ADHERENT STAINLESS STEEL COATING, AND MAINTAINING THE EFFECTIVE CARBON CONTENT OF SAID STRIP AT NOT MORE THAN 0.01% DURING SAID SINTERING. 